Efficiency and cost-effectiveness of copper electrowinning is and for a long time has been important to the competitiveness of the copper industry. Past research and development efforts in this area have thus focused—at least in part—on mechanisms for decreasing the total energy requirement for copper electrowinning, which directly impacts the cost-effectiveness of the electrowinning process.
Conventional copper electrowinning, wherein copper is plated from an impure anode to a substantially pure cathode with an aqueous electrolyte, occurs by the following reactions:
Cathode reaction:Cu2++SO42−+2e−→Cu0+SO42 (E0=+0.345 V)
Anode reaction:H2O→½O2+2H++2e− (E0=−1.230 V)
Overall cell reaction:Cu2++SO42−+H2O→Cu0+2H++SO42−+½O2 (E0=−0.855 V)
Conventional copper electrowinning according to the above reactions, however, exhibits several areas of potential improvement for, among other things, improved economics, increased efficiency, and reduced acid mist generation. First, in conventional copper electrowinning, the decomposition of water reaction at the anode produces oxygen (O2) gas. When the liberated oxygen gas bubbles break the surface of the electrolyte bath, they create an acid mist. Reduction or elimination of acid mist is desirable. Second, the decomposition of water anode reaction used in conventional electrowinning contributes significantly to the overall cell voltage via the anode reaction equilibrium potential and the overpotential. The decomposition of water anode reaction exhibits a standard potential of 1.23 Volts (V), which contributes significantly to the total voltage required for conventional copper electrowinning. The typical overall cell voltage is approximately 2.0 V. A decrease in the anode reaction equilibrium potential and/or overpotential would reduce cell voltage, and thus conserve energy and decrease the total operating costs of the electrowinning operation.
One way that has been found to potentially reduce the energy requirement for copper electrowinning is to use the ferrous/ferric anode reaction, which occurs by the following reactions:
Cathode reaction:Cu2++SO42−+2e−→Cu0+SO42− (E0=+0.345 V)
Anode reaction:2Fe2+→2Fe3++2e− (E0=−0.770 V)
Overall cell reaction:Cu2++SO42−+2Fe2+→Cu0+2Fe3++SO42− (H0=−0.425 V)
The ferric iron generated at the anode as a result of this overall cell reaction can be reduced back to ferrous iron using sulfur dioxide, as follows:
Solution reaction:2Fe3+→SO2−+2H2O→2Fe2++4H++SO42−
The use of the ferrous/ferric anode reaction in copper electrowinning cells lowers the energy consumption of those cells as compared to conventional copper electrowinning cells that employ the decomposition of water anode reaction, since the oxidation of ferrous iron (Fe2+) to ferric iron (Fe3+) occurs at a lower voltage than does the decomposition of water. However, maximum voltage reduction—and thus maximum energy reduction—cannot occur using the ferrous/ferric anode reaction unless effective transport of ferrous iron and ferric iron to and from, respectively, the cell anode(s) is achieved. This is because the oxidation of ferrous iron to ferric iron in a copper electrolyte is a diffusion-controlled reaction. This principle has been recognized and applied by, among others, the U.S. Bureau of Mines Reno Research Center and Sandoval et al. in U.S. Pat. No. 5,492,608, entitled “Electrolyte Circulation Manifold for Copper Electrowinning Cells Which Use the Ferrous/Ferric Anode Reaction.”
Although, in general, the use of the ferrous/ferric anode reaction in connection with copper electrowinning is known, a number of deficiencies are apparent in the prior art regarding to the practical implementation of the ferrous/ferric anode reaction in copper electrowinning processes. For example, prior embodiments of the ferrous/ferric anode reaction in copper electrowinning operations generally have been characterized by operating current density limitations, largely because of the inability to obtain a sufficiently high rate of diffusion of ferrous iron to the anode and ferric iron from the anode. Stated another way, because these prior applications have been unable to achieve optimum transport of ferrous and ferric ions to and from the anode(s) in the electrochemical cell, prior applications of the ferrous/ferric anode reaction have been unable to cost effectively produce copper cathode in electrochemical cells employing largely conventional structural features.
Another aspect of the prior art that would benefit from additional innovation relates to the configuration and composition of anode design to help optimize the ferrous/ferric anode reaction. For example, dimensionally stable electrodes for use in electrowinning of metals generally consist of a base or substrate of a valve metal, typically titanium, carrying an electrocatalytic coating such as a mixed oxide of platinum group metal and a valve metal forming a mixed crystal or solid solution. Many different coating formulations have been proposed. The state of the art with titanium mesh anodes is to place a precious metal oxide or a valve metal oxide coating on the mesh to serve as the anode conductive coating. These coatings typically are very expensive. An anode coating that achieves the benefits of prior art electrocatalytic coatings, but that also offers cost savings, would be advantageous. Alternatively, anodes for use in connection with the ferrous/ferric anode reaction comprised of less expensive materials—such as, for example, carbon composite materials and stainless steels—that perform similarly to traditional anodes but at reduced cost, would be advantageous.